1. Field of the Invention
The present invention relates to an automatic transmission for a vehicle, and more particularly relates to an automatic transmission wherein three single pinion type planetary gear sets are used to construct a gear train.
2. Description of the Related Art
As is known, planetary gear sets have three components: a sun gear, a ring gear, and a carrier for holding a pinion gear meshing with them. One of these components is used as an output member, another is used as an input member and the other element is held stationary, whereby the input rotation may be increased in output speed, reduced in speed in the forward direction, or reduced in speed in the reverse direction. Therefore, in general, in the related art, a plurality of planetary gear sets are combined to construct a gear train for an automatic transmission. In this case, the transmission ratio obtained varies depending on the method of combination of the planetary gear sets, the value of the gear ratio of the planetary gear sets (ratio of number of teeth of sun gear and ring gear), and whether use is made of the single pinion type planetary gear set or use is made of the double pinion type planetary gear set. Not all of the combinations, however, may be used in practice. Due to various conditions such as the mountability in the vehicle, the feasibility of manufacture, the gear change characteristics, and the drive performance sought, there are limited gear trains which can be used in practice. In other words, a massive number of constructions of gear trains are possible through various combinations of the planetary gear sets and settings of the gear ratios, so creation of one satisfying the conditions sought for an automatic transmission for a vehicle is accompanied with great difficulty.
In the past, proposals have been made of numerous automatic transmissions conceived of under this situation. Among these, an automatic transmission using three single pinion type planetary gear sets to construct a gear train has been disclosed, for example, in Japanese Published Unexamined Patent Application Nos. 60-88252 and 60-57036.
The automatic transmission disclosed in Japanese Published Unexamined Patent Application No. 60-88252 is constructed of a combination of three single pinion planetary gear sets and can set five forward speeds and one reverse speed as transmission stages. If, to prevent a remarkable reduction in the drive power of the vehicle before and after gear shifts, the gear ratio of the planetary gear sets is set to give a relationship where the gear ratio of the transmission stages is close to an equal ratio series, it is necessary to set the gear ratio of one of the planetary gear sets to a remarkably high value. As a result, it is inevitable to make the outer diameter of the planetary gear set large. Further, if the gear ratios of the opposing planetary gear sets are set so that the outer diameters do not increase particularly greatly, the relationship will not be one where the gear ratios of the transmission stages are close to the equal ratio series and there is the problem of causing the inconvenience of a deterioration in the power performance of the vehicle.
In the automatic transmission disclosed in Japanese Published Unexamined Patent Application No. 60-57036, the sun gear of a first planetary gear set and the sun gear of a second planetary gear set are normally or selectively connected, the sun gear of the first planetary gear set and the carrier of the second planetary gear set are connected via a clutch, the ring gear of the second planetary gear set and the ring gear of a third planetary gear set are connected, the carrier of the first planetary gear set is connected to these ring gears and the carrier of the second planetary gear set and the sun gear of the third planetary gear set are connected via a clutch. An input shaft is connected via a clutch to the sun gear of the first planetary gear set and the sun gear of the second planetary gear set, which are mutually connected themselves, while they are connected via another clutch to the ring gear of the first planetary gear set. An output shaft is connected to the carrier of the third planetary gear set. As a brake means for stopping the rotation, provision is made of a brake which holds stationary the mutually connected sun gear of the first planetary gear set and the sun gear of the second planetary gear set and a brake which holds stationary the sun gear of the third planetary gear set. In the automatic transmission according to Japanese Published Unexamined Patent Application No. 60-57036, in the first gear, the brake for holding stationary the sun gear of the third planetary gear set is engaged and simultaneously the clutch connecting the carrier of the second planetary gear set to the sun gear is engaged, whereby the rotation of the carrier of the second planetary gear set is inhibited. Further, even in reverse gear, the carrier of the second planetary gear set is held stationary in the same way. The transmission ratio between the first gear and reverse gear becomes a similar value.
Note that Japanese Published Unexamined Patent Application No. 60-57036 mentioned above shows by a skeleton view a construction wherein a one-way clutch is inserted between the carrier of the second planetary gear set and the case, but a corresponding operation table for the same is not shown and, further, no explanation is made of the settable transmission stage, so it would be considered that transmission stages would be set in the same way in a construction where the one-way clutch is additionally provided as with the other examples disclosed in this publication.
Therefore, in the automatic transmission disclosed in Japanese Published Unexamined Patent Application No. 60-57036, to stop the rotation of the carrier of the second planetary gear set, the carrier is connected by the clutch to the sun gear of the third planetary gear set and the brake for holding stationary the sun gear of the third planetary gear set is engaged with a brake. As a result, a large torque is applied to the clutch connecting the carrier and sun gear, so the clutch must be made large in capacity. Along with this, there is the problem that the automatic transmission becomes larger in size. Further, in the automatic transmission disclosed in Japanese Published Unexamined Patent Application No. 60-57036, in the second gear or the second' gear or third' gear, the sun gear of the first planetary gear set is held stationary, but in this case, the brake for holding stationary the sun gear of the second planetary gear set is engaged and the sun gear of the first planetary gear set is engaged with that sun gear by a clutch, so a large torque is applied to the clutch in these transmission stages and, as a result, the clutch must be made large in capacity. This is a problem in that it becomes a factor enlarging the size of the automatic transmission.